Photothermographic elements are known from the past and described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. The photothermographic elements generally have photosensitive layers which contain a reducible silver salt (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a reducing agent, and optionally a toner for controlling the tone of silver, typically dispersed in an binder matrix. After imagewise exposure, photothermographic elements are heated at an elevated temperature (e.g., 80.degree. C. or higher), whereby redox reaction takes place between the reducible silver salt (functioning as an oxidizing agent) and the reducing agent to form a black silver image. This redox reaction is promoted by the catalysis of a latent image of silver halide produced by exposure. The black silver image is thus formed in the exposed area.
Thermography or heat development has the advantage of easy and quick processing because it eliminates a need for processing solution as used in the wet development process. However, the image forming process by wet development is still the mainstream in the photographic art. This is because the thermographic process yet leaves an outstanding problem which never occurs with the wet development process.
It is a common practice to add dyes to photographic photosensitive materials as a filter or for the anti-halation or anti-irradiation purpose. Specifically, the dye is added to a non-photosensitive layer and exerts its function upon imagewise exposure. If the dye having exerted its function is left in the photographic photosensitive material, the image formed therein can be colored with that dye. Therefore, the dye must be removed from the photographic photosensitive material during development. In the wet development process, the dye can be readily removed from the photographic photosensitive material into the processing solution. By contrast, the thermographic process is very difficult or substantially impossible to remove the dye.
A simple and quick development process is required for the modern photographic art, especially in the fields of medical photography and printing photography. Improvements in the wet development process, however, have approached the plateau. For this reason and others, engineers in the fields of medical photography and printing photography now pay attention to the thermographic image formation.
With respect to the problem of difficult removal of dyes in the thermographic process, it was proposed to decolorize the dye by the heat applied during heat development. For example, U.S. Pat. No. 5,135,842 discloses that polymethine dyes of a specific structure can be decolorized by heating. U.S. Pat. Nos. 5,314,795, 5,324,627, and 5,384,237 disclose that polymethine dyes are heated for decolorization using carbanion generators.
Some of the prior art methods, however, are short in decolorization, failing to reach a sufficient degree of transparency within the desired time. Some dyes are quickly bleachable, but a problem is left with respect to the water resistance of processed images. When the processed elements are incidentally contacted with water droplets or stored in high-humidity conditions, a loss of transparency occurs, resulting in alterations.